Cytidine deamination to deoxyuridine is a major source of mutation in both the germ-line and cancer cells. Base excision and mismatch repair pathways remove deoxyuridine from double stranded DNA. However, this lesion occurs commonly in single-strand (ss) DNA formed during replication, transcription, and after exposure to DNA damaging agents. Mammalian cells also express the AID/APOBEC family of enzymes that catalyze cytidine deamination in ssDNA and whose aberrant activity has been linked to carcinogenesis. Like the ssDNA they target, expression of APOBEC enzymes is induced by a variety of environmental agents suggesting these exposures may have an important role in carcinogenesis by increasing both APOBEC enzyme and substrate levels. The extent to which APOBEC cytidine deaminases damage nuclear DNA, their potential synergism with environmental DNA damaging agents, and the contribution of deamination-induced mutation to cancer etiology are unknown. Also unclear is how deaminated cytidines are processed within ssDNA. The goal of this proposal is to characterize mechanisms and sources of cytidine deamination-induced mutation and their possible contribution to environmentally-induced cancer. AIM I will address if the number of APOBEC-induced mutations in human cancers correlates with clinical measures of disease progression and prognosis. The alteration of cancer-related genes by APOBEC-induced mutations will also be determined. AIM II will determine how conversion of deoxyuridine to abasics sites in ssDNA impacts mutagenesis and ectopic recombination. Deamination-induced mutation and recombination will be measured in yeast lacking damage tolerance mechanisms to access these pathways' roles in mutation and translocation avoidance. AIM III will address the mutagenic capacity of APOBECs on human chromosomal DNA in a cell line model. The impact environmental exposures that induce ssDNA formation and/or APOBEC expression will be addressed. Whole genome sequencing will be used to identify genome features at risk of APOBEC-mediated mutation. These aims will provide an understanding of cellular and environmental mechanisms governing cytidine deamination-induced mutation and their contribution to carcinogenesis.